The present invention relates generally to devices and methods for performing gamma correction of a display, and more particularly to devices and methods for accurately modeling an inverse gamma curve used to turn on and off pixels in a ferroelectric liquid crystal (FLC) display while maintaining an average voltage of zero volts or a null voltage across the FLC display.
In an analog based pixel cell architecture, a pixel value representing the brightness of a pixel is stored as an analog voltage rather than in a digital representation. The higher the stored voltage corresponding to the pixel value, the brighter the pixel is. A more detailed description of analog based pixel cell architectures is provided in U.S. application Ser. Nos. 09/070,487 and 09/070,669, which are incorporated herein by reference.
In this analog pixel cell architecture, a control signal called Vramp is routed throughout the pixel array. The_Vramp signal is a monotonically increasing signal. Initially, the pixel is in the xe2x80x9conxe2x80x9d or_display state. When the voltage of the Vramp signal rises to a level equal to the voltage stored inside the pixel, the pixel output switches to the xe2x80x9coffxe2x80x9d state in which the pixel does not display. Each pixel in the array of pixels of a display will switch from xe2x80x9conxe2x80x9d to xe2x80x9coffxe2x80x9d at a time relative to its own stored voltage.
If a first pixel stores, for example, twice the voltage as a second pixel, then the first pixel will be on for twice as long as the second pixel if the Vramp signal rises at a constant rate. A problem with this method is that doubling the length of time that the first pixel is on as compared to the second pixel does not mean the first pixel will appear twice as bright as the second pixel.
Briefly, in one aspect consistent with the present invention, a method for generating a Vramp signal to be applied to pixels of a display selects a first step value from a plurality of step values and adds the first step value to the Vramp signal for each of a first number of clock cycles. A second step value is selected from the plurality of step values and is added to the Vramp signal for each of a second number of clock cycles, different from the first number of clock cycles. The sample points used to determine the step values are preferably chosen to be close enough together that the slope over any region between sample points is not significantly different from the average slope, but far enough apart that memory is not wasted storing nearly identical values.
In another aspect consistent with the present invention, a method for generating a Vramp signal applied to pixels of a display selects sample points based upon an inverse gamma curve having a selected gamma factor. An average slope between each consecutive pair of selected sample points is then determined. A step value is calculated for each determined slope. One of the calculated step values is then added to the Vramp signal at each clock cycle. Each of the calculated step values is used for more than one clock cycle.
In a further aspect consistent the present invention, the Vramp signal is generated in an illumination period and in a balance period. In yet a further aspect of the present invention, the Vramp signal is applied to the pixels of the display such that each pixel of the display is in an on-state during the illumination period for the same amount of time each pixel is in an off-state during the balance period.